This invention relates to measuring apparatus and more particularly to measuring apparatus for determining the internal diameter of the heat transfer tubes in a nuclear steam generator.
A typical nuclear steam generator comprises a vertically oriented shell, a plurality of U-shaped tubes disposed in the shell so as to form a tube bundle, a tube sheet for supporting the tubes at the ends opposite the U-like curvature, and a dividing plate that cooperates with the tube sheet forming a primary fluid inlet plenum at one end of the tube bundle and a primary fluid outlet plenum at the other end of the tube bundle. The primary fluid having been heated by circulation through the nuclear reactor core enters the steam generator through the primary fluid inlet plenum. From the primary fluid inlet plenum, the primary fluid flows upwardly through first openings in the U-tubes near the tube sheet which supports the tubes, through the U-tube curvature downwardly through second openings in the U-tubes near the tube sheet, and into the primary fluid outlet plenum. At the same time, a secondary fluid, known as feedwater, is circulated around the U-tubes in heat transfer relationship therewith, thereby transferring heat from the primary fluid in the tubes to the secondary fluid surrounding the tubes causing a portion of the secondary fluid to be converted to steam. Since the primary fluid contains radioactive particles and is isolated from the secondary fluid by the U-tube walls and the tube sheet, it is important that the U-tubes and tube sheet be maintained defect-free so that no breaks will occur in the U-tubes or in the welds between the U-tubes and the tube sheet, thus preventing contamination of the secondary fluid by the primary fluid.
Occasionally, one or more of the heat transfer tubes may become internally dented due to a variety of possible reasons. One such reason for the internal denting is thought to be corrosion on the outside of the tube which presses in on the tube and causes the internal flow area of the tube to become constricted. Not only does this denting phenomenon restrict the flow through the particular heat transfer tube, but it also may lead to a failure in the tube at the point of denting which may allow the primary fluid to leak into the secondary fluid thus contaminating the secondary fluid. Of course, contamination of the secondary fluid must be prevented; therefore, such a dented tube must be repaired or replaced before the denting results in a crack in the tube. Since the denting phenomenon is normally a localized problem, before a particular tube can be repaired or replaced it is necessary to determine which tube or section of a tube suffers from the denting problem. Therefore, what is needed is a measuring device that is capable of being inserted into and traversing a constricted tube and yet capable of proper self-alignment within the tube such that the extent of denting and the location of the denting with respect to the longitudinal length of the tube may be determined.